Noise suppression



Sept.12,193 9. F.J.HOOVEN ETAL 2,172,558

NOISE SUPPRESSION Filed Nov. 21, 1936 BY Mi.

23:24am ATTORNEY.

Patented Sept. 12, 1939 UNITE YATES P OFFHCE NOISE SUPPRESSION Delaware Application November 21, 1936, Serial No. 112,031

8 Claims.

This invention relates to the reception of radio signals and more particularly to a method and means for suppressing noise in the output of a radio receiver.

It is well known that the ignition system of a gasoline engine causes electrical disturbances in the tuned circuits of an adjacent receiver. It is also well known that similar disturbances are caused by atmospheric conditions, commutators of electric motors, and by many types of high voltage electrical apparatus in use today. Many of these disturbances are exceedingly brief in duration but set up secondary impulses which are sustained for comparatively long periods and are commonly called noise, because of the effect they have in the loud speaker or telephones of a receiving set. These disturbances are often of no particular frequency but are lumps of electrostatic energy of irregular wave form and of high ceived. Although their duration is ordinarily short, their eifect is sustained for a suflicient period to distort signals that are received with and immediately after the noise impulse. When these disturbances are reproduced in the audio frequency apparatus, they cause mechanical vibrations in the loud speaker or telephone that mask the signals entirely and render them unintelligible.

One of the objects of the present invention is to provide an improved circuit means constituting a noise reducer system for a radio receiver that will function equally well on weak or strong signals without adjustment.

Another object of the invention is to provide means whereby the efiects of random noise on a radio receiver are reduced without distortion of a properly modulated signal.

Another object is to provide an efficient method of eliminating the harmful effects of noise due to electrical disturbances in the output of a receiver.

A still further object is to provide a noise reducing circuit that can be installed in a con- -Ventional radio receiver without adjustment at the time of installation or during subsequent use,

in order to operate properly on weak and strong signals.

A further object is to provide a modulation limiting circuit whereby the output of a radio receiving circuit may be maintained substantially unaffected by modulation amplitudes in excess of a predetermined value.

A further object is to provide means in an automatic volume control system whereby large,

amplitude as compared to the signals being re-- intermittent noise impulses may be prevented from affecting the sensitivity of a receiver.

Other objects of the invention include the provision of not only means of demodulating the signal of excessive modulation percentages, but also of determining the percentage of modulation at which these means become effective independently of the voltage of the received signal, and also the provision of a noise reducing circuit that will require a minimumof parts to be added to the receiving circuit.

A full understanding of the invention and its Various uses may be had from the following specification and from the drawing, in which,

Figs. 1 to 4 represent typical oscillograms for different wave conditions in receivers as heretofore employed;

Figs. 5 and 6 represent oscillograms of waves, the shapes of which have been modified by use of the present invention; and,

Fig. 7 is a diagram of a radio receiver circuit embodying the present invention.

Most of the noise interference encountered in radio reception is of a highly damped, intermittent nature. The noise impulse themselves are of extremely short duration but are of such relatively large amplitude that their maintenance and slow damping in a circuit completely overshadow the desired signal.

Fig. 1 is taken from an actual oscillogram of a modulated signal having superimposed upon it the interference from a gasoline engine ignition system and illustrates the short duration and relative large amplitude of a typical source of radio interference. The signal in Fig. lrepresents modulation at about ninety per cent by a frequency of eight hundred cycles per second, while the noise impulses represent a frequency of one hundred per second, which latter frequency represents the sparking that occurs in an eightcylinder, four-stroke engine running at 1500 R. P. M. Fig. 2 represents the amplifier output of the signal shown in Fig. 1 prior to detection in the absence of noise. Fig. 3 represents the eifect of noise on a signal in the output of an amplifier employing automatic volume control. The type of automatic volume control commonly used provides a circuit by means of which the output of the amplifier is rectified and made to produce a negative grid bias on the amplifying tubes, causing a reduction in sensitivity that is proportional to the signal voltage. This reduction in sensitivity is represented by the varying amplitudes of the wave peaks shown in Fig. 3. When the noise impulses of high amplitude are received,

ordinarily the automatic volume control system causes the sensitivity to be reduced to such a degree that the desired signal is of low audibility, and the output of a typical linear detector under these circumstances will appear as shown in Fig. 4, wherein the only audible component of sound will be that of the interfering signal.

If means are provided whereby the radio receiver is prevented from responding to impulses greater than the maximum modulation peaks of a properly modulated carrier, the output of the amplifier will appear as represented in the oscillogram shown in Fig. 5. When detected, such a signal appears as shown in Fig. 6, and it will be observed that the signal is substantially unaffected by the noise impulses.

Means have heretofore been provided whereby the response of a radio receiver may be limited to a predetermined voltage, but it has always been necessary when using such means to adjust either the sensitivity of the receiver or the value of the limiting voltage, or both, so that the limiting voltage is substantially equal to the peak modulation of the received signal. Since few automatic volume control circuits are so effective as to provide constant output voltage regardless of the input, it follows that for each new signal, or for each time a signal fades, it is necessary to readjust either the gain of the receiver or the limiting voltage. The weak signals are most adversely affected by noise and are constantly fading. Therefore, such automatic means: require frequent adjustment, and the failure to perform such adjustment results in either the unintelligible distortion of the strong signals or the failure to limit noise when receiving weak signals.

In the noise suppressing circuit of the invention, one embodiment of which is shown in the drawing, means are provided whereby the limiting output voltage of the receiver is constantly maintained as a fixed predetermined function of the average carrier output voltage, such average being integrated over a period of one-hundredth to onehalf a second, regardless of the absolute magnitude of the carrier output.

In a normal modulated signal of one hundred per cent modulation, it may be assumed that the maximum peak signal will not exceed twice the average signal and that a circuit which arbitrarily limits the peak of the signal output to twice the average will not distort a one hundred per cent modulated signal. The embodiment of the invention shown in Fig. 7 includes an automatic volume control means commonly found in receivers, and, although the present invention is not restricted to such sets, it greatly improves the operation of the automatic volume controls found in so many of the sets now in use. The invention not only is used for eliminating noise from the audio-frequency output, but is highly useful in modifying the operation of the usual automatic volume control circuits. It can be seen that the presence of the conventional automatic volume control circuit is not necessary to the operation of the invention, but as is pointed out below, the

invention can be used to prevent the conventional automatic volume control from reducing the sensitivity of the receiver. Those skilled in the art can perceive this readily from the embodiment shown when taken in conjunction with the following description.

As will be readily understood from Fig. '7, the signal is received in an ordinary antenna circuit and selected and then amplified in the pentode I.

.The Voltage appearing across coil 2 is impressed on anode 4 of a double diode tube 3, the cathode 5 of which is connected to ground through the usual resistance 6. The rectified signal current flows through resistances 6 and I, the latter of which has a value considerably higher than resistance 6, thus causing condensers 8 and 9 to be charged negatively with respect to ground. Condenser 8 and resistor I are so proportioned that their impedances are equal at some frequency above the highest modulation or audio frequency to be received, but lower than the carrier frequency. Condenser 8 affords a high frequency ground for the carrier frequency voltage. Resistor I and condenser 9 are similarly proportioned, and the audio voltage will be impressed on the grid of the triode II, the anode of which may be connected to any suitable audio output device. Resistor I2 charges condenser I3 to the same average negative voltage, which is substantially the average peak signal voltage as that appearing across condenser 8 and resistor 'l, but resistor I2 and condenser I3 are so proportioned that their impedances are equal at a frequency lower than the lowest modulation frequency intended to be received so that the charge on condenser I3 remains at the average carrier peak voltage or at the peak voltage of the unmodulated signal. This voltage is then applied back to the antenna circuit 2 in a well-known manner so that the grid of the pentode I is maintained at a potential negative with respect to ground and the cathode of pentode I, which potential is proportional to the value of the amplified signal, thus varying the conductance and therefore the amplification of tube I according to the signal voltage.

In the circuit just described, the noise reducer system is used as. discussed below. The cathode I of tube 3 is connected to the coil 2 at approximately its center point through the condenser I4. Cathode I5 is connected to ground through resistor I6 which is so proportioned to condenser I4 as to have an impedance equal to condenser I4 at a frequency higher than the highest modulation frequency, so that radio frequency voltage will exist on cathode I5 across resistor I6, but audio frequency voltage will exist across condenser I4, the latter having low impedance to high frequencies but not to modulation frequencies.

Whenever the instantaneous signal exceeds double the average signal, which condition exists when the modulation percentage exceeds one hundred, it follows that half the signal will exceed the average so that the cathode I5 will become momentarily negative with respect to the charge on condenser I3, thus causing current to fiow to anode ll of the tube 3 through resistors I8 and I6. Resistor I8 has a high impedance relative to resistor I6 and therefore most of the rectified voltage appears as negative charge on the condenser I9, which is of high impedance relative to resistor I8 at audio frequencies. This negative charge is impressed on the grid of the triode 20 through condenser 2I across a grid leak 22, condenser 2I being of low impedance as compared with resistor 22 at audio frequencies. The anode of tube 20 is connected to the usual B battery through high resistance 23 so that the anode voltage of tube 20 is low in the absence of a signal. However, when a negative bias is applied to the grid of tube 20, the anode current is decreased and the anode assumes a more positive potential which is impressed upon cathode 5 of tube 3 through the condenser 24 across the resistor 6, which resistor has a high impedance 75..

at audio frequencies relative to condenser 24. When this positive. bias is. impressed on cathode 5, it follows that there is adecrease in the voltage across resistor 1 and condenser 8, thus reducing the voltage applied to the grid of triode II. The positive bias on cathode 5 has the additional function of preventing the charging of condenser I3 by sudden increases of the carrier, thereby preventing a lowering of the sensitivity of the receiver, which decrease in sensitivity has heretofore followed upon operation of the automatic volume control circuit, as has been above pointed out. The result of such operation of the automatic volume control produces the effect illustrated in Fig. 3. This lowering of sensitivity is inherent in the automatic volume control devices now in use because these devices depend on the negative bias that is impressed on the radio amplifier, such as pentode I in Fig. 7, for limiting excessive voltages in the set, but as can be readily seen, the bias suflicient to limit noise of high volume, for example of 200 per cent of normal modulation, will drive the grid of the amplifier very negative causing it to be blocked. This would not be very objectionable if it were the only result, but as has been stressed already, the effects of the noise are sustained for relatively long periods, and the conventional automatic volume control does not eliminate these effects, but merely lowers the receiver sensitivity in an amount measured by the amplitude of the disturbance, thereby causing loss, in the audio output, of the weaker signals. The embodiment of the invention illustrated in Fig. '7 shows how, by preventing condenser |3 from being negatively charged in response to noise impulses, which negative charge would result were the conventional automatic volume control used alone, the noise impulses are thereby prevented from lowering the receiver sensitivity.

It should be noted, however, that should the increase in carrier amplitude be of long duration instead of instantaneous, the charges on condensers 2i and Z l will leak off through the associated resistors 22 and 6, thus permitting the cathode 5 to resume its normal operating potential and preventing the blocking of the receiver which might otherwise occur. It should also be noted that carrier increases up to two hundred per cent of average, such as would occur in a normal one hundred per cent modulated signal, will not cause the noise suppressor to operate, due to the fact that the voltage impressed on cathode I5 is only half that impressed on anode 4 because of the center tap on coil 2 which will cause operation of the noise suppressor only when the carrier exceeds two hundred per cent of normal unmodulated voltage. It is possible, however, by changing the radio frequency voltage on cathode E5 of tube 3, to cause the noise suppressor to operate on any desired function of the average carrier, thus suppressing the modulation peaks of any desired modulation percentage. This might be efiected by placing the connection of cathode l5 at some point other than the center of coil 2. It can be seen that with the suppressor set to become operative at modulation percentages of one hundred per cent and over, the center tap of coil 2 will cause cathode I5 to become negative and effect suppression of the noise, but by choosing any point along coil 2 as the tapping point, cathode i5 may be caused to assume a negative bias with respect to the charge on condenser 2| and to effect suppression of noise of any desired percentage of normal modulation. It

can be stated, therefore, that noise of a predetermined characteristic is suppressed, as we know both the general type of noise wave and range of percentages that can be suppressed by use of the present invention.

It will be readily seen, from the above description, that there has been provided an effectual yet simple means for eliminating the effects of noise of any predetermined percentage of modulation above 100 per cent modulation, without the troublesome effect of decreasing the sensitivity of the signalin the operation of the noise suppressor. This is a very important advantage over the volume control devices of the prior art, for the lowering of the sensitivity of a receiver, as is discussed above, may result in the audible detection only of noise, and the signal, or parts of it, may be of such low amplitude as to be indiscernible. It will also be readily seen that by employing means, as above pointed out, for preventing the blocking of the receiver in the presence of a comparatively long noise train, we have avoided a common defect that is present in most auto matic volume control circuits of the prior art.

While the invention has been shown in only one embodiment, it is clear that it is not restricted to this showing. It has been above stated that the invenion is not restricted to receivers employing automatic volume control means. By disconnecting the conductor labeled AVC lead, the receiver will function without automatic volume control. It is clear that the modulation limiting property of the invention will not be impairedby the absence of automatic volume control. Further, it will be readily understood that the operation of the triode i l with fixed grid bias, instead of its bias being determined by the carried voltage, as shown, will not be impaired. The noise suppressor, therefore, can be used entirely apart from the automatic volume control circuit shown, or any other ordinary control. Fig. 7, and its explanation, show, however, that the noise suppressor circuit exerts an ingenious control over the conventional automatic control by its added ability to prevent the decrease in receiver sensitivity that would occur without the use of the noise suppressor. Reference will be had primarily to the appended claims for a definition of the limits of the invention.

What is claimed is:

1. In a system for receiving modulated carrier waves, a space discharge amplifier comprising a control electrode and having its output connected to a thermionic rectifier through an inductance, said rectifier having two anode-cathode circuits, one of said circuits having means for controlling the gain of said amplifier in accordance with the negative charge on a condenser, said condenser being charged in accordance with the amplitude of the carrier, the cathode of the first of said circuits being connected to said inductance intermediate its ends and being adapted to assume a negative bias upon reception of instantaneous modulation above a predetermined limit, a triode having a grid adapted to be negatively biased in response to said modulation, the anode of said triode being connected to the other of said anodecathode circuits, said triode having a normally low anode voltage which increases as said grid is biased negatively to effect a positive bias on the cathode of said other circuit, for biasing the grid of an output tube negatively and simultaneously preventing said instantaneous modulation from affecting the sensitivity of the receiver immediately after the reception thereof.

2. In an automatic volume control system for the reception of modulated carrier waves, an amplifier of the space discharge type comprising a control electrode, the output of said amplifier being inductively coupled to a thermionic rectifier circuit, means responsive to variations in carrier amplitude for impressing a Variable negative bias on said control electrode, means responsive to a predetermined level of instantaneous modulation for impressing a negative bias; on the cathode of said rectifier, a triode having a grid biased negatively in response to changes in said cathode potential, the anode circuit of said triode being connected to a second rectifier circuit, and an output amplifier of the space discharge type having a variable negative grid bias in response to changes of potential of said second rectifier anode.

3. In a noise suppressor system for radio receivers, a rectifier having a signal input, means subject to noise impulses of amplitudes greater than signal voltages in said input for causing a change in bias of the cathode of said rectifier, an audio output tube, and a second rectifier, a thermionic discharge device having cathode, anode and control grid electrodes in circuit with said second rectifier and means in circuit with the said cathode and anode electrodes for blocking said audio output tube upon instantaneous reception of noise impulses.

4. In a radio receiver of the class having amplifier means, the output of which is detected and fed back to the amplifier for controlling the volume of the said output, the combination with said amplifier of a rectifier circuit for detecting noise, said circuit including a triode having a grid biased negatively in accordance with the amplitude of the noise, the anode of said triode being normally at low potential, a condenser, means for charging the condenser negatively with respect to ground in response to increases of the potential of said anode, an audio amplifier and means for impressing the charge of said condenser on the grid of said audio amplifier to control the volume of the audio output.

5. In a radio receiving device having a radio frequency amplifier, audio amplifier, and a space discharge device associated therewith, the method of suppressing noise of a predetermined percentage of instantaneous modulation which comprises "amplifying the modulated carrier, detecting the output thereof, feeding back the rectified output according to the amplitude of the modulated carrier to bias said amplifier input negatively and thereby detect noise of a predetermined level or over, controlling the bias of said space discharge device by means of said detected noise, rectifying the resultant voltage and impressing the output thereof on the input of said audio amplifier to cause periods of low audio output coincident with the reception of noise impulses and preventing a reduction in sensitivity of said amplifier to the carrier during periods subsequent to instants of noise impulse reception.

6. In a receiver of carrier frequency energy having radio and audio frequency amplifiers and an automatic volume control, the method of suppressing noise of a predetermined characteristic which comprises amplifying the carrier energy, detecting the output thereof, feeding back the rectified output according to the amplitude of the modulated carrier to impress a variable negative bias on said radio amplifier input and thereby control the volume of said carrier, detecting said noise, utilizing said detected noise to lower the output of said audio amplifier so as to instantaneously damp the noise impulse and preventing the reduction of sensitivity of the radio amplifier for a predetermined interval subsequent to the reception of a noise impulse.

'7. In a device of the class described, an input circuit subject to the reception of signal and other alternating currents of varying amplitude, means for rectifying said alternating currents, a thermionic device including a control grid and an anode, means for operatively connecting said anode to said rectifying means, means responsive to the reception of current of an amplitude above signal current and of a predetermined characteristic for increasing the potential of said anode, an output amplifier having a control grid and an anode, means responsive to said increase in anode potential for impressing a negative bias on said last-named grid, and output means operatively secured to the second-named anode.

8. In a noise suppressor system, input and output thermionic amplifiers, rectifier means intermediate said input and output amplifiers, gain control means in circuit therewith for controlling the gain on said input amplifier, additional rectifier means responsive to a predetermined voltage amplitude for blocking said output amplifier, and means associated with said additional rectifier means for disabling said gain control means while said output amplifier is blocked.

FREDERICK J. HOOVEN. CARL G. SERIGHT. 

